1. Field of the Invention
The present invention relates to an electrode for an electric double-layer capacitor, and a slurry for forming the electrode.
2. Description of the Related Art
There is a conventionally known electric double-layer capacitor made using a solution of quaternary ammonium borofluoride compound in propylene carbonate as an electrolyte. Vapor-activated carbon having a turbulent structure is used as activated carbon in electrodes of the electric double-layer capacitor, namely, positive and negative electrodes.
With the electric double-layer capacitor provided with the positive and negative electrodes made from the activated carbon described above, a relatively large electrostatic capacity can be provided, but in order to meet the demand as an energy source for a hybrid-type automobile, for example, a further increase in electrostatic capacity is desired.
To meet such demand, the present inventors have made various studies, and as a result, they have reached the consideration of alkali-activated carbon made from meso-phase pitch as a starting material for use as activated carbon for the positive and negative electrodes. The alkali-activated carbon has a graphite structure of a good electric conductivity incorporated in a surface of particles of the activated carbon, thereby realizing the maintaining of a high density and a low resistance. With the electric double-layer capacitor including the positive and negative electrodes made using such alkali-activated carbon, the electrostatic capacity can be increased remarkably more than that in the conventionally known capacitor.
However, as a result of studies further advanced, it has been found that the rest potential of the alkali-activated carbon in an electrolyte is offset relatively largely toward an oxidizing reaction region from an ideal rest potential of the activated carbon, and for this reason, there is a possibility that an oxidizing reaction may be liable to occur in the positive electrode during charging, whereby the durability of the electric double-layer capacitor may be impeded.
There is a conventionally known electrode for an electric double-layer capacitor, which includes an active material and a conductive material and is connected to a current collector.
When the resistance of contact between the current collector and the electrode is high, an increase in internal resistance of the electric double-layer capacitor is brought about. Therefore, to decrease the contact resistance, for example, the following measures are conventionally employed: a measure of subjecting a current collector made of an aluminum foil to an etching treatment to increase the contact area, and a measure of bonding an electrode to a current collector by an adhesive containing a conductive filler and then subjecting the collector and the electrode to burring or punching to enhance the close contact between the collector and the electrode.
However, the conventional measures are incapable of decreasing the contact resistance to an expected extent even with a high working cost. Therefore, a further decrease in contact resistance is desired.
Further, there is a conventionally known electrode for an electric double-layer capacitor, which is formed using a slurry under utilization of a doctor blade process.
Such a slurry conventionally contains CMC (sodium carboxymethyl cellulose) added as a thickening agent in addition to activated carbon, a conductive filler and a binder.
It may be contemplated that activated carbon made from meso-phase pitch as a starting material, i.e., meso-phase activated carbon is used as the above-described activated carbon to aim at an increase in electrostatic capacity. However, the following problem is encountered: This meso-phase activated carbon is liable to be agglomerated in a slurry, because it has a high water-absorbing ability, and when an agglomeration has been produced, the thickness and density of the formed electrode are non-uniform, resulting in decreases not only in performance but also in strength of the electrode.